Automatic Gain Control (AGC) is used in virtually all radio receivers, and serves several functions. Primarily AGC is necessary to keep signal levels within a range so that a system's amplifiers are not overloaded. AGC serves to maintain proper tracking loop gain for servos and maintain levels that are appropriate for demodulation. Many modulation formats, such as 64QAM and 16QAM, need to adjust the signal level fairly accurately in order for the demodulator to deliver a usable data stream.
Prior art AGC has been limited because it has been implemented with an analog servo loop or feedback loop. For example, a signal may arrive at the demodulator and feed back to the input stages to adjust the gains of the input stages. This prior art system is undesirable because if an impulse comes through the system, a radar pulse for example, it is too late for the AGC to act upon the impulse. The high power signal has already clipped the data transmission as it goes through the amplifiers before the AGC can ever see it to try to turn down the gain. As a result there is damage to the wave form, and thousands of bits of data are destroyed before the AGC can get around to setting a new level that would accommodate the radar pulse.
Currently, there are several so-called “last mile” and “last foot” data transmission systems which are designed to deliver high speed and/or high data capacity from one location to another. Several such systems use RF transmission to replace copper wire or coaxial cable. Some of these systems are called point to point or point to multipoint systems and operate in the 28-38 GHz bands. A fundamental characteristic of such existing systems is that their RF transmissions occur in a frequency spectrum protected and regulated by a government body. These protected frequency spectrums, or bands, are licensed to certain license holders and only one (or a selected few) may operate in any given physical area. In such situations, rigorous rules apply to anyone holding permits for the usage of those protected bands. Another fundamental characteristic of such protected bands is that all users are licensed to perform the same type of RF transmission.
When operating in a licensed band the interference between transmissions is not only homogeneous, i.e., wideband, it originates from the same type of antenna to accomplish the same type of transmission and is thus controllable. Accordingly, noise (interference from another transmitter on the same frequency or on an interfering frequency) typically will be evenly spread.
In a typical licensed application, the frequency coordination would mathematically predict a certain low level of interference. If a system could not achieve a low level of interference, a license would not be granted for most systems. Once the governing body grants the license, all those holding licenses to the same band are afforded protection. Thus, in a protected band, if a narrow band interferer is detected, the licensed user could call the FCC (or other policing agency) and ask that the agency investigate and rectify the problem. In an unlicensed band, the user is essentially on his/her own and usually no such official remedy is available.
Because of the licensed nature of such RF bands, only a limited number of companies may provide service within those bands. Thus, in order to widen the choices consumers have, it is desirable for service providers to be able to use unlicensed RF bands to provide high data rate capability to deliver high speed, high capacity data services.
In 1997 the FCC created a wireless arena called Unlicensed National Information Infrastructure (U-NII). System operators are free to operate wireless equipment in three sub-bands (5.15 to 5.25 GHz, 5.25 to 5.35 GHz and 5.725 to 5.825 GHz) without acquiring a licensed frequency spectrum. Part 15 of the FCC document specifies the conditions for operating wireless equipment in the U-NII frequency band. However, operators are not protected from possible interference from other U-NII operators transmitting in the vicinity or even other systems which utilize the same frequencies.
The IEEE, a standards group, is defining a wireless LAN standard, referred to as IEEE 802.11a for operation in the U-NII band. Equipment that conforms to this standard will operate indoors at the lower frequency sub-band i.e. 5.15 to 5.25 GHz. The ETSI BRAN group in Europe has defined an air interface standard for high-speed wireless LAN equipment that may operate in the U-NII frequency band. Equipment that is compatible with this standard may cause interference with use of these unlicensed bands.
One major problem with the use of such unlicensed bands is that it is very difficult, if not impossible, to control RF interference from other users of the unlicensed band. These other users may be using the selected unlicensed band for uses which are essentially different from that employed to deliver communication services. For example, the 5.25 to 5.35 GHz and 5.725 to 5.825 GHz bands are available for use for outdoor data communication between two points. This is typically a wideband use. The same bands are also available for use by radar. When the same band is used for point to point communication, and also used by others such as radar, data communications between sending and receiving antennas will experience significant interference from radar pulses, which are broadcast over a wide area in small (narrow) repetitive bursts.
In the current state of the art, there is no discrimination between continuous or repetitive interference. When interference is detected, it is usually based on a signal to noise ratio for any given channel, then the radio switches to a lower level modulation, from either 64QAM to 16QAM, or 16QAM to QPSK, or QPSK to BPSK. For orthogonal frequency division multiplexing (OFDM), the modulation order of the subcarriers is reduced. Lower modulation shifting allows more tolerance for noise and interference.
Prior art radar interference mitigation is intended for use in currently licensed RF bands. However, radar interference is not an issue of great concern in licensed bands because there is little or no such interference. Most licensed bands are free and clear of other harmful interferers. Additionally, most unlicensed bands do not have strong radar interferers. However, there is other low level interference in the unlicensed RF bands. This interference is at a much lower level and has a different signature than high powered radar. Therefore, generally speaking, prior art interference mitigation systems do not detect radar interference nor do they attempt to avoid it.
As described above, prior art AGC is analog. It basically has feedback loops so it monitors what it receives, but by the time it responds to dampen a signal, data is lost. The prior art AGC is not sufficient for use with RF data transmission in relatively noisy unlicensed bands. A large amount of in-band interference, and potentially high power interference, such as radar pulses are present in unlicensed RF bands which is very disruptive to high speed data transmissions in those bands.
In an unlicensed RF band data transmission environment it is difficult to determine a proper AGC bandwidth. If the AGC bandwidth is too low, response is very slow. The AGC will miss fast rising events. If the AGC bandwidth is too high then it will track things that happen quickly. However, the AGC may overcompensate, lowering gain levels too quickly or to too low of a level, resulting in lost data. Still, the AGC must respond quickly enough that, when there is a legitimate fade of the signal due to interference, it will track the interference event.
Prior art AGC will not allow both high gain and low gain settings at the same time. An AGC circuit that has a very fast response time will, when it is exposed to a radar pulse, tend to ramp very quickly to a new higher or lower level in order to accommodate the radar pulse. Ramping very quickly will either raise the gain to an unacceptable level or lower the gain disrupting the transmission rate, which results in a signal which cannot be properly modulated/demodulated. Ideally to accommodate a radar pulse, it is desirable to identify it as a radar pulse and be able to predict its occurrence and its duration.